APPLIED PHYSICS LETTERS 89, 033901 共2006兲
Optimized fabrication and electrical analysis of silver nanowires templated
on DNA molecules
Sung Ha Park, Matthew W. Prior, Thomas H. LaBean,a兲 and Gleb Finkelsteinb兲
Department of Physics, Duke University, Durham, North Carolina 27708; Department of Computer Science,
Duke University, Durham, North Carolina 27708; and Department of Chemistry, Duke University,
Durham, North Carolina 27708
共Received 1 February 2006; accepted 19 June 2006; published online 21 July 2006兲
We report on the electrical conductivity measurement of silver nanowires templated on native
␭-bacteriophage and synthetic double-stranded DNA molecules. After an electroless chemical
deposition, the metallized DNA wires have a diameter down to 15 nm and are among the thinnest
metallic nanowires available to date. Two-terminal I-V measurements demonstrating various
conduction behaviors are presented. DNA templated functional nanowires may, in the near future,
be targeted to connect at specific locations on larger-scale circuits and represent a potential
breakthrough in the self-assembly of nanometer-scale structures for electronics layout. © 2006
American Institute of Physics. 关DOI: 10.1063/1.2234282兴
Recent developments in DNA-based nanotechnology
have shown the suitability of this novel assembly method for
constructing useful nanostructures.1,2 DNA molecules can
serve as precisely controllable and programmable scaffolds
for organizing functional nanomaterials3,4 in the design, fabrication, and characterization of nanometer scale electronic
devices and sensors.5 DNA templated metallic nanowires are
an example of the capability of DNA scaffolds and have been
considered an interesting research subject since 1998, when
first reported by Braun et al.6 Since then, DNA has been
metallized with silver,7 gold,8 palladium,9 platinum,10 and
copper.11 Up to this point most templates for fabricating
nanowires have used native bacteriophage ␭-DNA molecules. Recently we proposed to use complex self-assembled
superstructures composed of DNA “tiles”1,2 as scaffolds for
templating functional nanoelectronic devices.12 Specifically,
we have reported on three distinct self-assembled onedimensional DNA nanostructures: cross-tile nanoribbons,13
triple-crossover nanotubes,14 and three-helix nanobundles15
and successfully demonstrated the use of these artificially
designed DNA nanostructures as nanowire scaffolds. In this
letter, we present the base sequence design of synthetic
double-stranded DNA 共dsDNA兲 and fabrication of metallic
silver nanowires templated on both synthetic dsDNA and
bacteriophage ␭-DNA molecules. We also demonstrate the
electrical conductivity of our metallized nanowires. While
nonmetallized DNA molecules do not appear to be highly
conductive,16 the DNA templated metallic nanowires promise to become useful as programmable interconnects in bioelectronic devices.
The DNA base sequence of the synthetic unit dsDNA tile
was designed to minimize the chance of sequence symmetry
and undesired associations.17 The strand sequence for the
molecules used here is given in Fig. 1. A unit dsDNA molecule consists of 50 nucleotides 共nts兲, has length ⬃16.2 nm,
and was designed to concatenate and form long doublehelical DNA. Synthetic oligonucleotides were purchased
from Integrated DNA Technology Inc. and purified by polya兲
Electronic mail: thl@cs.duke.edu
Electronic mail: gleb@phy.duke.edu
b兲
acrylamide gel electrophoresis 共PAGE兲. Complexes were
formed by mixing a stoichiometric quantity of each strand in
standard buffer, 1 ⫻ TAE/ Mg++ 关40 mM Tris acetate 共pH
8.0兲, 2 mM EDTA, and 12.5 mM magnesium acetate兴. Oligo
mixtures were cooled slowly from 95 to 20 ° C in 2 l of
boiled water in a styrofoam box for two days to facilitate
hybridization. Incubation of annealed samples at 4 ° C overnight prior to examination by atomic force microscopy
共AFM兲 improved the quality of the imaging data.
Native ␭-DNA 共Promega Inc.兲, about 50 000 base pairs
in length, with a concentration of 1.6 nM and synthetic dsDNA molecules of 1.0 ␮M were visualized by tapping mode
AFM in air and under buffer, respectively. For AFM imaging
in air, samples were prepared by pipetting ␭-DNA solution
共⬃20 ␮l兲 onto a mica substrate, they were allowed to adhere
for 5 min, rinsed gently by doming a drop of water onto the
mica, then dried under a stream of nitrogen. For AFM imaging in liquid phase, ⬃5 ␮l sample was spotted on mica and
left to adsorb to the surface for 5 min. Then, 30 ␮l of buffer
was placed onto the mica and another 30 ␮l of buffer was
pipetted onto the AFM tip. Imaging was performed under
buffer in a tapping mode fluid cell on a Multimode NanoScope IIIa 共Digital Instruments兲 using NP-S tips 共Veeco
Inc.兲. Figure 2共a兲 is an AFM image of ␭-DNA and 共b兲, synthetic DNA. From the inset in Fig. 2共b兲, we see the helical
pitch of the DNA molecules with a peak-to-peak distance of
3.4± 0.3 nm, in good agreement with the known distance of
⬃3.4 nm. The length of the synthetic dsDNA varies from a
FIG. 1. 共a兲 DNA base sequences of synthetic dsDNA molecules. The arrows
indicate simplified strands running from 5⬘ to 3⬘. The complementary sticky
end of a is a⬘. 共b兲 An atomic-resolution cartoon of a unit dsDNA molecule
which consists of 50 base pairs with 46% CG content.
0003-6951/2006/89共3兲/033901/3/$23.00
89, 033901-1
© 2006 American Institute of Physics
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033901-2
Park et al.
FIG. 2. 共a兲 AFM image of ␭-DNA in air. 共b兲 AFM image of synthetic
dsDNA in liquid phase. 共Inset兲 In a high resolution AFM image with a
periodic bump pattern on the dsDNA. 共c兲 and 共d兲 are height measurements
of single ␭-DNA 共⬃0.7 nm兲 in air and synthetic dsDNA 共⬃1.1 nm兲 molecules under buffer. 共e兲 and 共f兲 are SEM images of ␭- and synthetic-dsDNA
after two-step silver metallization process.
few hundreds of nanometers to several microns with average
length of ⬃5 ␮m. Representative section profiles of single
␭-DNA height of ⬃0.7 nm and synthetic dsDNA of
⬃1.1 nm are shown in Figs. 2共c兲 and 2共d兲. Empirically, we
noticed that the tapping mode AFM height of single layer
duplex DNA molecules were 0.6± 0.2 nm in air18 and
1.2± 0.2 nm under buffer where the known diameter of the
double-helix DNA molecules was ⬃2 nm. These heights are
in good agreement with the ones measured previously under
similar conditions.
We applied a two-step metallization process19 to coat
␭-DNA and synthetic dsDNA molecules in silver. Study of
lengths and diameters of silver nanowires, formed on dsDNA, had shown their dependence upon the times of incubation of the DNA with glutaraldehyde treatment both at
room temperature and on ice, time of dialysis, and time of
incubation of DNA with the initiator AgNO3. We have found
that the optimal conditions yielding nanowires with average
lengths of 3 ␮m and average widths of 35 nm can be obtained from the following parameter values; DNA sample
was incubated with 0.2% glutaraldehyde in 1 ⫻ TAE/ Mg++
buffer on ice for 30 min, then at room temperature for
20 min, then DNA sample was loaded into a Slide-A-Lyzer
Mini Dialysis unit, and dialyzed for 15 h at 4 ° C in 1 l of
1 ⫻ TAE/ Mg++ buffer 共we describe the complete procedure
in Ref. 19兲. Figures 2共e兲 and 2共f兲 are scanning electron microscopy 共SEM兲 images of ␭- and synthetic-dsDNA templated silver nanowires. The metallized nanowires display
widths down to 15 nm and lengths up to 7 ␮m. We observed
that the nanowires width 共as measured by SEM兲 and height
Appl. Phys. Lett. 89, 033901 共2006兲
FIG. 3. 共a兲 Linear two-terminal I-V characteristics of silver nanowires. 共Inset兲 Initial high resistance can be reduced by applying bias voltage. The
white arrow indicates voltage sweep direction. After the first few sweeps of
bias voltages, current dramatically changes at a certain critical voltage Vc
共here, Vc = −0.9 V兲. 共b兲 The I-V curves of a silver nanowire measured at two
different temperatures, 300 and 77 K. 共Inset兲 SEM image of an actual device
with scale bar, 100 nm. Cr–Au double layer electrodes, with 5 nm of Cr
followed by 25 nm of Au, were patterned by electron beam lithography onto
the nanowires on the silicon substrate.
共as measured by AFM兲 agree with one another within ⬃10%.
We next performed two-terminal I-V measurements of
the silver nanowires templated on both ␭- and syntheticdsDNA molecules. In many samples, the as-prepared wire
resistance was very high and dramatically decreased following several voltage sweeps in the range of about ±3 V 关inset
of Fig. 3共a兲兴. Most probably the initial high resistance is
caused by an oxide or contamination layer covering the
nanowire, which prevents formation of an adequate contact
between the nanowire and the metal leads. No visible granularity was observed in the SEM 关Figs. 2共e兲 and 2共f兲兴, so a
weak coupling between grains along the length of the wires
seems to be an unlikely explanation for the initial highly
resistive state. The oxide or contamination layer is evidently
destroyed upon application of a large enough source-drain
voltage Vc. The magnitude of Vc in different nanowires varies from a few millivolts to roughly 3 V. The two-terminal
I-V curves of about 30 共out of 70兲 samples in the lowresistance state show mostly linear behavior. The samples
presented in Fig. 3共a兲 demonstrate resistances of 895 ⍀ for a
first ␭-DNA templated nanowire 共␭1兲, 597 ⍀ for a second
sample 共␭2兲, 798 ⍀ for a first synthetic dsDNA templated
nanowire 共synthetic 1兲, and 784 ⍀ for a synthetic 2 measured at 0.1 V. These numbers correspond roughly to bulk
resistivities 共␳兲 of ⬃20, 10, 5, 4 ⫻ 10−6 ⍀ m for ␭1, ␭2, synthetic 1, and synthetic 2, respectively. The resistivity of bulk
silver is 1.6⫻ 10−8 ⍀ m. We observed that larger Vc typically
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033901-3
Appl. Phys. Lett. 89, 033901 共2006兲
Park et al.
The authors thank Jie Liu in Duke Chemistry Department for providing access to the Nanoscope IIIa AFM and
Alena Karpusenka for helpful discussion. This material is
based upon work supported in part by the U.S. Army Research Laboratory and the U.S. Army Research Office under
Grant No. W911NF-05-1-0466 and NSF Grant Nos. EIA-0218376 and CCR-03-26157.
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19
DNA sample was incubated with 0.2% glutaraldehyde in 1 ⫻ TAE/ Mg++
buffer on ice for 30 min, then at room temperature for 20 min, then the
DNA sample was loaded into a Slide-A-Lyzer Mini Dialysis unit 共Pierce,
Rockford, IL兲, and dialyzed overnight at 4 ° C in 1 l of 1 ⫻ TAE/ Mg++
buffer. A 50 ␮l sample of aldehyde-derivatized DNA was moved to a new
test tube. Then the initiator AgNO3 共solution A, HQ Silver™-EM Formulation, Nanoprobes Inc.兲 共50 ␮l兲 was added into the aldehyde-derivatized
DNA sample and left for 10 min in the dark. Silicon substrate was treated
with 1% aminopropyltriethoxysilane 共APS兲 prior to DNA sample deposition for better adhesion to the substrate. Then 15 ␮l of the silver seeded
DNA sample was deposited onto silicon substrate, allowed to adsorb for
10 min, then excess reagent was rinsed off with diH2O, and dried under a
stream of nitrogen. In the second step, HQ Silver™-EM formulation was
used according to the manufacturer’s instruction. One unit of initiator 共A兲
was mixed with one unit of moderator 共B兲 and one unit of activator 共C兲.
Then 15 ␮l of this fresh mixture was pipetted onto the sample on the
silicon substrate and left for 10 min for further metallization. Finally, excess reagent was rinsed off again with diH2O and dried under a stream of
nitrogen.
20
X. Y. Qin, L. D. Zhang, G. S. Cheng, X. J. Liu, and D. Jin, J. Phys. D 31,
24 共1998兲.
1
2
FIG. 4. Nonlinear two-terminal I-V characteristics of silver nanowires.
About 15% of total number of measured nanowires are non-Ohmic and may
have slightly asymmetrical I共V兲 with respect to zero bias. Among them,
three data sets, two from ␭ indicated L1 and L2 and the other S from
synthetic-dsDNA templated wires, are shown.
resulted in larger resistance 共R兲 after the increase in conductance that followed the application of high current. Finally,
Fig. 3共b兲 shows I-V curves of the ⬃35 nm width nanowire
with ⬃110 nm between electrodes and shows R of ⬃500 ⍀
at 300 K and ⬃30 ⍀ at 77 K. This change of resistance
共⬃17-fold for a change in temperature of about 4-fold兲 is
unexpectedly large, compared to the available data on
nanostructured silver films of a significantly higher
conductivity.20
We also note that about 10 nanowires out of 70 showed
non-Ohmic I共V兲 behavior after the initial increase in conductance. In these cases, the oxide/contamination layers between
electrodes and nanowires might not be removed thoroughly
during applying bias voltages even at voltages higher than
Vc. After applying voltages higher than 5 V, most nanowires
were damaged severely and become disconnected. The range
of biases where the conductance was suppressed varied from
a few millivolts to ⬃6 V. Three typical I-V nonlinearities are
shown in Fig. 4.
In conclusion, we have presented a method for fabrication of metallic silver nanowires, templated on ␭-DNA and
synthetic dsDNA molecules, which have been formed by a
two-step chemical deposition of silver. The nanowires display uniform widths down to ⬃15 nm and lengths up to
7 ␮m; these results are easily reproducible. We have also
demonstrated two-terminal I-V curves of silver nanowires.
Understanding of the mechanism limiting the preinitialized
conductance of the wires is necessary before these DNA templated wires can be reliably used as interconnects in bioelectronic nanodevices.
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